1. Field of the Invention
The present invention relates to the control of self excited alternating current induction motors and more particularly to the torque control of such motors.
2. Description of the Prior Art
Self excited alternating current motors, for example, induction motors, possess the highly desirable qualitites of design simplicity and mechanical ruggedness. These attributes suggest the ready application of such motors in commercial and industrial apparatus. However, control of the output condition, such as the speed, torque, or power, of induction motors in the manner necessary for such applications is difficult and has substantially limited their usage.
To some extent, the difficult in controlling such motors results from the same mechanical simplicity which increases their utility. Self excited alternating current motors consist of a flux producing stator winding distributed in a stator core surrounding a cage of rotor current conductors and shorting end rings embracing a rotor core. There is no direct electrical connection to the rotor, as by commutators, slip rings or the like. While the absence of commutators or slip rings and brushes eliminates the numerous problems associated therewith, the lack of an electrical connection to the rotor also prevents direct control of electrical conditions in the rotor. The electrical condition of the rotor can be controlled only indirectly by the enrgization of the stator winding.
Further, the speed of an alternating current motor is dependent, in great part, on the frequency of the alternating current applied to the motor and in the usual case, the most readily available power source is a constant frequency, constant voltage power source, for example, 60 Hertz, 240 volt power mains. Operation of an alternating current motor from such a power supply severely limits its application since the output characteristic exhibited is that of constant speed operation.
To provide variable speed operation to alternating current motors, variable frequency, variable voltage power supplies have been utilized. While the use of such power supplies permits control of the output condition of the motor, severe problems have been encountered in establishing and maintaining the desired output condition. This prevents attainment of truly satisfactory operation. These problems arise, in part, because small errors in the magnitude and phase displacement of the voltage applied to the motor can cause large errors in the current drawn from the variable voltage power supply and large errors in the output condition of the motor, making control of the applied voltage extremely critical.
Other problems occur when the operative condition is varied and arise because of the complexity of the operative relationships occurring in the motor under such circumstances. These transient phenomena include factors such as the leakage reactance time constant of the motor, the flux time constant of the motor, and the mechanical time constant of the rotor and the motor load. The occurrence of these conditions make mathematical analysis of the phenomena and the motor output condition difficult and impede the design of a satisfactory electric regulator of the voltage control type.
The present invention overcomes the shortcomings of such prior art motor controls by means of a motor control method, the operation and structure of which may be characterized as current controlling. This is in contradistinction to the prior art motor controls of the voltage type. While current controls have been described, such controls are not commonly known or used at present. The article by F. Blaschke et al. in the German publication "SiemensZeitschrift", Vol. 42, No. 9, pages 773-777 (1968) shows a motor control of the current type controlling a self excited alternating current motor. However, the described control is extremely complex in structure and control technique, requiring numerous feedback loops which defeat the inherent simplicity of current controls, hereinafter noted. Further, the described control is subject to an oscillatory condition which prevents the attainment of satisfactory operation.
The advantage of motor control of the current type over those of the voltage type reside in the fact that electric motors are essentially torque producing devices and that current, rather than voltage, is the basic torque producing quantity. While voltage may be related to current and thus to motor torque, the relationship between voltage and current is a complex one involving the motor leakage reactance and other phenomena. The complexity of this relationship accounts for the above-mentioned difficulty in obtaining satisfactory operation by controlling the applied voltage.